Special field in table

Hello All,

I need to create a new table and one of its field has a special property. This field have to inform automatically when new record is created and its value have to be secuencialy for each table key. For instance, The table key is the invoice number. When the first new record for the invoice number 00000001 is created, the field record number should be 001, the next should be 002,...

For the invoice number 00000002, when the first new record is created, the field record number should be 001, then 002,...

Can somebody help me?

Thank you in advance

Manuel